The present invention relates to a current limiting device for electromagnetically suppressing an overcurrent in an a.c. current path.
In order to protect an electric apparatus from an overcurrent caused by an interline short circuit or ground short circuit in a current path of, e.g., a distribution line, an overcurrent flowing through the electric apparatus is required to be limited at once to a certain low value. Of current limiting devices for limiting an overcurrent, the device of the type using a superconductive coil is known as disclosed, e.g., in Japanese Patent Laid-open No. 48-2038 (1973).
FIG. 1 shows the structure of the current limiting device disclosed in the above-mentioned publication. In this current limiting device, a low temperature container 53 with a conduit 54 for a cooling medium such as helium is provided. In the container, coils 51 and 52 made of superconductive material are wound about a common spool. In a current path through which power is supplied to the coils, there are connected in series an ordinary conductive reactor 55 and a switch 59. The directions of magnetomotive forces generated by the coils 51 and 52 are arranged to be opposite to each other, and the critical current values of the coil 51 is set lower than that of the coil 52.
The operation of the above-described current limiting device will be described below. In an ordinary current operation, both the coils 51 and 52 are in a superconductive state so that there is only a leakage flux .PHI.s between the coils, i.e., only a leakage reactance component, to thereby allow a low loss power supply. In contrast, when an overcurrent appears in a current path 58, the coil 51 having a lower critical current value quenches first and changes into an ordinary conductive state to have a high resistance. The current flowing through the coil 51 is therefore limited by the high resistance, and almost all the current is transferred to the coil 52 which is in a superconductive state. Magnetic flux passing through the coil 52 is generated and a corresponding inductance is produced therein. This inductance component suppresses the overcurrent. Thereafter, an operator opens the switch 59 to completely disconnect the current path. This current path disconnection is performed in order to suppress further consumption of expensive cooling medium (e.g. helium) caused by evaporation due to heat generation by the coil 51.
However, the switch is required to be opened by the operator so that a time delay of disconnection poses the problem of consuming the cooling medium.
Another current limiting device is also known as disclosed in Japanese Patent Laid-open Publication No. 63-253315 (1988) wherein about the outer circumferential periphery of a superconductive coil wound non-inductively, there is wound a superconductive reactor having a larger critical current value than that of the superconductive coil.
This current limiting device however is not satisfactory in that the non-inductive superconductive coil or trigger coil has a large loss at the quenching state depending on the value of the quenching resistance if the system voltage is high, thereby deteriorating the superconductive wire due to heating, or in some cases burning or melting the wire.
It is therefore necessary that the quenching resistance value of the trigger coil be made larger as the system voltage becomes higher. However, according to conventional technology, the necessary larger quenching resistance value results in a bulky current limiting device. A compact device has thus been desired.
Further,, after the current limiting operation is carried out after an accident has occurred in the system and the cause of the accident is removed, it is also desired to recover the ordinary state of the current limiting device as soon as possible. However, with the conventional technology, if a trigger coil quenches once, the recovery to the superconductive state thereof requires a cooling period on the order of at least one second after the disconnection of the current path. Until such a cooling period is completed, the current limiting device as a whole cannot recover its ordinary state.
The current limiting device of the type shown in FIG. 1 is generally used in suppressing a short circuit current in a current path of high voltage and large current. For example, such a current limiting device is applied to each feeder of a power distribution unit having rated values of 6.6 KV and 600 A. In this case, the requirements of the current limiting device are that the current limiting operation starting current is 1200 A, the operating impedance is 2 ohms, and the current limiting peak value is about 3 KA. In order to satisfy such requirements, the inductance of the current limiting coil 52 should be about 6.5 mH for the 50 Hz system. In addition, the coil is required to be made of a superconductive wire having a critical current value larger than 3 KA so that the structure of the coil becomes necessarily of a multi-layer wound type.
With the current technology, the structure of the coil 52 has the following dimensions, for example. The inner diameter is 100 mm, the outer diameter is 200 mm, the coil length is 40 mm, and the number of coil turns is 400, constructed of ten layers. In order to minimize the leakage reactance (ordinary impedance) of the device, the other coil 51 is disposed under the coil 52 and constructed to have a magnetomotive force substantially the same as that of the coil 52 and opposite in direction.
The rate of cancelling magnetic fluxes of the coils 51 and 52, however, decreases as the difference between coil cross sections becomes large. The above-described illustrative device has a leakage inductance of about 1 mH which acts as the ordinary impedance of the current limiting device, resulting in a drop of the supply voltage. For example, about a 190 V voltage drop will be generated by the above-described current limiting device having the rated values of 6.6 KV and 600 A.
As described above, with a conventional superconductive current limiting device, the higher the voltage of the system circuit is, the larger the operating impedance (impedance of the coil 52) required, resulting in an increase of the leakage inductance and ordinary voltage drop.